SAN DIEGO WATER HAS AN AFRICAN CONNECTION

Point Loma 
The Scripps Institution of Oceanography in La Jolla has found evidence that some of the water consumed in San Diego County can be traced to dust and microbes that swirl out of the Sahara Desert and cross the Pacific, where they help trigger snowstorms in the Sierra Nevada and Cascade mountains. The snow later melts and flows to reservoirs, where it is distributed to about one-third of the state’s residents, including those in San Diego.

Scientists knew that wind carries dust and other aerosols from Asia to California. But the new Scripps study shows for the first time that dust and microbes drift to the Sierra Nevada from as far away as the Sahara in northern Africa, a distance of more than 8,000 miles.

The report, published in the journal Science, said the dust and microbes turn into ice above the Sierra, contributing to the formation of snowstorms. The findings are largely based on atmospheric readings taken in February 2011. Researchers identified aerosols above the Sierra that had been present over Oman 10 days earlier, and that had probably originated over the Sahara several days before then.

As it moved east, the Saharan dust mixed with aerosols from China and Mongolia. Scripps scientists said this mixture seeded clouds above the Sierra.

Researchers have yet to determine whether the aerosols greatly enhance snowfall. They plan to pursue that question because many California communities depend on runoff from the Sierra.

“We need to quantify the effect of that dust and use that information in weather models,” said Kim Prather, an atmospheric scientist at Scripps and lead author of the study.

Runoff is especially important to San Diego, which has very little groundwater. The county uses about 200 billion gallons of water a year, about 80 percent of it imported from the Sierra Nevada and the Colorado River.